libGDX: Determination of the direction of touchDragged

Question

When I write my code inside touchDragged method to allow the player dragg with any one of these following directions:

( up, down, right, left, up-right, up-left, down-right, down-left )

My code:

public void touchDragged(InputEvent event, float x, float y, int pointer) {
    for (int i = 0; i < 4; i++) {
        for (int j = 0; j < 4; j++) {
            Image img = img[i][j];
            float imgX = img[i][j].getX();
            float imgY = img[i][j].getY();
            float width = img[i][j].getWidth();
            float height = img[i][j].getHeight();
            if (x > imgX && x <= imgX + width && y > imgY && y <= imgY + height) {
                img.setColor(Color.RED);
            }
        }
    }
});

The result: (When the player draggs in up-right direction)

But How can I prevent below squares that contains X to listen the touch event? because I know the player want to go this direction up-right.

Do I calculate first the slope of straight line linear equation then according to the slope I determine the direction? or what?

Any Idea ??